Influenza is a common infectious disease of the respiratory system associated with the Orthomyxoviridae family of viruses. Because of the high degree of variability of the virus, vaccination is typically required on a yearly basis with a reformulated vaccine that takes into account strain variations. The vaccine composition developed each year in the United States is determined by the Department of Food and Drug Administration Vaccines and the Related Biologicals Advisory Committee. The World Health Organization (WHO) similarly operates a global surveillance network of laboratories, for detection of new influenza variants, e.g., see Lavanchy, Vaccine 17: S24 (1999). Selection is based on antigenic analysis of recently isolated influenza viruses, the patterns of spread of antigenic variants, and the antibody responses of recently vaccinated individuals.
Influenza A and B are the two types of influenza viruses that cause epidemic human disease. Influenza A viruses are further categorized into subtypes on the basis of two surface antigens: hemagglutinin (HA) and neuraminidase (N). Influenza B viruses are not categorized into subtypes. Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses and influenza B viruses have been in global circulation. Vaccination is recognized as the single most effective way of preventing or attenuating influenza for those at high risk of serious illness from influenza infection and related complications. The inoculation of antigen prepared from inactivated influenza virus stimulates the production of specific antibodies. Protection is afforded only against those strains of virus from which the vaccine is prepared or closely related strains.
Each year's vaccine contains three virus strains (usually two type A and one type B) representing the influenza viruses that are believed likely to circulate in the coming winter. The antigenic characteristics of current and emerging influenza virus strains provide the basis for selecting strains included in each year's vaccine. The WHO reviews the world epidemiological situation annually and if necessary recommends new strains based on the current epidemiological evidence.
Despite the recomposition, it is not possible for a vaccine to include all the different strains actively infecting people in the world during a particular season. In addition, a relatively long length of time is also required to formulate and prepare sufficient quantities of vaccine doses for responding to seasonal increases in flu infections. Typically, it can take over six months to prepare a vaccine. As a result, new or overlooked influenza strains can become prominent during that six month period, leading to an epidemic. In April 2009 a novel influenza A (H1N1) virus of swine origin was first detected. It is a quadruple reassortant compared to the previously described H1N1 subtype. The initial outbreak of the virus was in Mexico but the epidemic has spread rapidly over the borders through the United States and Canada and now almost all international countries are reporting new cases. There remains a need in the art for improved compositions and methods for treating influenza. In particular, there is a need for compositions that have broad immunogenicity against both seasonal influenza strains recommended by WHO and against new emerging pandemic influenza strains such as the new influenza A (H1N1) virus of swine origin.